1. Field of the Invention
The present invention relates generally to a sidestream gas monitoring system for monitoring a patient's medical condition, and, in particular, to a filter assembly for use in a sidestream monitoring system that separates undesired liquid components from respiratory gases to be monitored.
2. Description of the Related Art
During medical treatment, it is often desirable to monitor and analyze a patient's exhalations to determine the gaseous composition of the exhalate. For instance, monitoring the carbon dioxide (CO2) content of a patient's exhalations is often desirable. Typically, the carbon dioxide (or other gaseous) content of a patient's exhalation is monitored by transferring a portion, or sample, of the patient's expired gases to a suitable sensing mechanism and monitoring system.
Monitoring of exhaled gases may be accomplished utilizing either mainstream or sidestream monitoring systems. In a mainstream monitoring system, the gaseous content of a patient's exhalations is measured in-situ in the patient circuit or conduit coupled to the patient's airway. In a sidestream monitoring system, on the other hand, the gas sample is transported from the patient circuit through a gas sampling line to a sensing mechanism located some distance from the main patient circuit for monitoring. As a patient's expired gases are typically fully saturated with water vapor at about 35° C., a natural consequence of the gas transport is condensation of the moisture present in the warm, moist, expired gases.
Accurate analysis of the gaseous composition of a patient's exhalation is dependent upon a number of factors including collection of a gaseous sample that is substantially free of liquid condensate, which might distort the results of the analysis. As an expired gas sample cools during transport through the gas sampling line to the sensing mechanism in a sidestream monitoring system, the water vapor contained in the sample may condense into liquid, i.e., condensate. The liquid, i.e., condensate, if permitted to reach the sensing mechanism, can have a detrimental effect on the functioning thereof and may lead to inaccurate monitoring results. Condensed liquid in the gas sampling line may also contaminate subsequent expired gas samples by being re-entrained into such subsequent samples.
In addition to the condensate, it is not uncommon to have other undesirable liquids, such as blood, mucus, medications, and the like, contained in the expired gas sample. Each of these liquids, if present in the gas sample to be monitored, may render analytical results that do not accurately reflect the patient's medical status.
There are numerous techniques for separating undesired liquids from the patient's expired gas stream to protect the gas sensing mechanism. For instance, it is known to place a moisture trap between the patient and the sensing mechanism to separate moisture from the exhalation gas before it enters the sensing mechanism. The challenge, however, is to achieve the separation without affecting the characteristics of the parameters being measured, e.g., the waveform of the gas to be monitored.
By way of example, carbon dioxide (CO2) is effectively present only in the patient's expired gases. Therefore, the CO2 in an exhaled gas sample, transported through a gas sampling line to the sensing mechanism, fluctuates according to the CO2 present at the point at which the sample is taken. Disturbances in this fluctuation, i.e., in the CO2 waveform, are undesirable because any such disturbance can affect the accuracy of the CO2 measurement and the graphical display of the waveform. For this reason, removal of liquids from the exhaled gas sample is desirably accomplished in such a way that it does not substantially disturb the CO2 waveform. Unfortunately, conventional moisture traps often disturb the waveform to a substantial degree.
Various other techniques have been employed to filter the expired gas stream of the undesired condensate while attempting to permit the waveform to be transported undisturbed. Such techniques include absorbents for wicking condensate out of the gas, centrifugal filters, desiccants, hydrophobic membranes for filtering gases and hydrophilic membranes for filtering liquids. While each of these techniques has its advantages, each has its drawbacks as well. Each of the techniques listed above can remove a portion of the liquid condensate, but none is foolproof, except, perhaps for those using a hydrophobic filter. A consequence, a portion of the liquid condensate may still reach the sensing mechanism in many conventional filters. On the other hand, conventional filters that use a hydrophobic filter, can disturb the waveform beyond acceptable levels. Accordingly, a suitable filter that does not substantially disturb the waveform of expired gases and while substantially separating moisture from the gas sample and while minimizing deadspace would be advantageous.